Whey proteins have conventionally been used as foodstuffs because of the high nutrition value possessed by the whey proteins themselves, and also have widely been used as supplemental materials for foods such as binders, extenders, and water retention agents, wherein the properties of the whey protein such as emulsifying capability, foaming capability, and gelling capability are utilized. Among these, the high gelling capability makes the whey protein an advantageous composition modifier for raw meat and fish meat products. Thus, the gelling capability of whey proteins is closely related to the texture and water retention capacity of foods and is an extremely important factor. Generally, non-denatured proteins are denatured by heat and create mutual interactions such as a hydrophobic interaction between the protein molecules and an SH/SS exchange reaction between non-covalent bonds, such as ionic bonds and hydrogen bonds, whereas whey proteins are denatured and gelled by heat at temperatures of 60.degree. C. or higher. However, because the whey protein gel thus obtained is generally non-transparent and has only a small water retention capacity and a fragile structure, such a whey protein gel is not preferably used as a food material.
For these reasons, in order to improve the gel structure of whey proteins a partial heat-denatured whey protein solution produced by partially denaturing the whey protein by heating, or a solution obtained by drying this partial heat-denatured whey protein solution to produce a dried powder and re-dissolving the dried powder, are used to obtain a whey protein gel which has high water-retention capacity and excellent texture. The present inventors have conducted studies concerning the characteristics of whey proteins and have found that whey protein molecules, which are usually spherical, produce a soluble coagulate in which protein molecules are linked like chains if partially denatured by heating, and have further found that this soluble coagulate of whey protein can be effectively produced by preparing a whey protein solution with a concentration at which gelling does not occur by heating, and treating this solution with heat at 55.degree. C. to 120.degree. C. and for 1 to 120 minutes.
This soluble coagulate in which protein molecules of the whey protein are linked like chains arc hereinafter referred to as "soluble coagulate", and the whey protein which contains this soluble coagulate is referred to "partial heat-denatured whey protein". Although the partial heat-denatured whey protein does not gell in the soluble coagulate state, a highly elastic gel with high water retention capacity can be obtained by producing a solution of the partial heat-denatured whey protein by heating a whey protein solution at a concentration of 4-15% by weight, preferably 5-12% by weight, and at 55.degree. C. to 120.degree. C., preferably 65.degree. C. to 95.degree. C., or by producing a solution by, drying this solution, and re-dissolving the resulting dried powder, and then by adding a salt at low temperatures to these solutions (Japanese published unexamined patent application No. 64550/1993), by acidifying these solutions (Japanese published unexamined patent application No. 124067/1990), or by freeze-drying and thawing these solutions (Japanese published unexamined patent applications No. 280834/1991 and No. 27249/1991). In this manner, the structure of the gel can be improved by partially heat-denaturing the whey protein.
The molecules in the partial heat-denatured whey protein solution are associated by a spray drying treatment or a lyophilization treatment due to concentration by drying. As a result, the powder of partial heat-denatured whey protein produces a precipitate when re-dissolved. Thus, the solubility of the powder is not necessarily good. In addition, the solubility is further reduced if the dried powder is stored for a long period of time. Development of a partial heat-denatured whey protein which can be stored for a long time has therefore been desired.